In tunneling machines it is advantageous to provide at least one adjustable bearing assembly through which the shaft of the boring head passes and in which this shaft is journaled, this assembly frequently being referred to as a large or massive bearing and which is constructed so as to enable the shaft to be shifted in a direction transverse to the shaft axis, i.e. in a selected radial direction.
In such machines, a bracing system is generally provided from which the boring head is advanced into the rock or other subterranean face and a bearing arrangement which allows shifting in a direction perpendicular to the shaft axis permits the tunnel or a bore to be advanced somewhat eccentrically with respect to the bracing structure.
In the past, the hydraulic piston-and-cylinder arrangements which have been provided for such adjustable bearing assemblies have comprised three piston-and-cylinder units including two vertical units and a third unit located below and between the first two units to provide a radial displacement.
The three piston-and-cylinder units and their radial points of attack are also subjected to the reaction forces and both the vertical cylinder units and the lower cylinder unit must be dimensioned to take up horizontal force components.
A significant problem with such systems is that the establishment of a given degree of eccentricity with this arrangement cannot be limited to a selective operation of the cylinder since any movement to an eccentric point requires compensation of the contraction or extension of each cylinder unit and thus some displacement of fluid from each. In general, therefore the displacement is kinematically indeterminate.
If the hydraulic back pressure is not sufficient to resist the compensatory flow, the position setting may be undefined.
As a result, prior art three-point adjustment arrangements of this type have not proved to be fully satisfactory.